The present invention relates to heating devices, particularly circuits for controlling heating devices.
The use of positive thermal coefficient (PTC) elements in electric heating pads and blankets is well known. Typically, a PTC element comprises an electrically conductive PTC plastic material arranged between two conductors. If, however, one of the conductors in intimate contact with the PTC material breaks, arcing may occur. Since the heating wire used in heating pads and electric blankets is typically made very thin and flexible and is subjected to repeated flexing from use, conductor breaks in the heating wire are common. When a conductor break occurs, a line voltage can develop across the break causing an arc to jump across the break. Such an arc can raise the temperature of the PTC material to auto ignition, which can start a fire. If allowed to continue for an extended period of time (e.g., approximately 250 ms or more) such arcing will likely ignite a fire.
A safety circuit for preventing this condition from continuing and possibly causing a fire is described in U.S. Pat. No. 4,436,986 to Carlson. When the Carlson circuit detects a conductor break in the heating element, it generates a current surge that blows an input power fuse, thereby disabling the application of power to the heater. The fuse, however, must be sized to handle currents of two or three times the continuous current rating of the heater in order to accommodate the current inrush associated with the start-up characteristics of the PTC material. The Carlson circuit also relies on the fuse to deactivate the appliance in all possibilities of short circuits.
Typically, an adjustable bimetallic control switch is used to provide differing heat settings for PTC-based heating appliances. As current flows through the bimetallic element, the element heats up and bends due to the differential expansion of the metals incorporated in the element. The deflection causes the contacts to open and interrupt the current to the heater and the bimetallic element to cease bending. The bimetallic element then cools down until contact is again made and the cycle repeats. The deactivation of this type of electromechanical control is typically accomplished by blowing a fuse that is in series with the switch.
Modern electrical power controls use solid state switching devices such as silicon control rectifiers (SCR), power transistors, solid state relays and triacs. U.S. Pat. No. 4,315,141 to Mills describes a temperature overload circuit having a pair of solid state switches biased by a temperature sensitive capacitive element. In such control systems, a small signal controls the switching of larger load currents.
Logic integrated circuits or microprocessors can be used to control high-speed solid state power switching devices. Such processors are typically capable of operating at speeds many times the 50 or 60 Hz frequency of typical AC power sources. This capability makes it possible to control each AC cycle and perform switching as the AC power waveform crosses zero thereby lowering the noise generation associated with AC switching and improving efficiency. Microprocessors and logic ICs, however require programming, thereby adding a significant level of complexity, customization and thus cost.
U.S. Pat. No. 5,420,397 to Weiss et al. describes a microcontroller-based detection circuit for limiting arcing time by either disabling the microcontroller or switching off the power. An interruption in either the hot or neutral AC power conductors will signal the microcontroller and, after a short time period, the microcontroller enters a safety mode condition in which power to the PTC heater is turned off. In order to prevent repetitive arcing by continuously restarting the microcontroller, the safety mode is reset only by removing power and waiting a predetermined time interval. Repeated and prolonged arcing will cause the arc zone to heat up, such that the arc causes the PTC material to break down, creating a carbon conduction path contributing to the volatility of the fault.
Typically, electric blankets and heating pads can be disconnected from their control circuits to allow the electric blanket or heating pad to be washed. For safety purposes, if the control circuit is turned on before the heating element is connected or if the heating element is disconnected while power is applied, the control circuit should go into a safety mode and deactivate the application of power.
A problem with known control circuits is that a fault cannot be detected before full power is applied. This can be very dangerous, since as soon as full power is provided, arcing may occur, which could result in electrocution and/or fire. It is therefore desirable to provide the unit with some means for detecting a fault before full power is applied.
The present invention provides a circuit for protecting users against failures of PTC wire heating elements in electrical heating appliances. In an exemplary embodiment, a sub-circuit located in the heating device (e.g., blanket, pad) senses the voltage at the end of the PTC wire and provides a heating element status signal to a control circuit. The control circuit mixes the status signal with a sample of the power supplied to the PTC wire to provide a signal representative of the condition of the PTC wire. The circuit of the present invention preferably detects when the temperature control cycles power to the PTC wire to prevent false tripping. If the signal indicates the presence of a fault, power is removed from the PTC heating wire. Power is kept off until the circuit is reset.
In a further exemplary embodiment, the sensing circuit uses phase in-coding to allow more than one sense circuit to use the same signal line, thereby reducing the number of conductors between the heating device and the control circuit. The heating device thus can be coupled to the circuit with a small number of conductors (e.g., 3 wires for a single heating element and 4 wires for two heating elements).
In an exemplary embodiment, the circuit of the present invention can be reset by removing power from the circuit (e.g., unplugging the appliance from a wall outlet). If power is reapplied while the fault is still present, the circuit will not reset.
The circuit of the present invention monitors each cycle of the AC power applied to the heating elements, thereby providing a fast response time for detecting intermittent failures before they develop into dangerous conditions.
Moreover, the common failure mode of the circuit components will cause a trip condition, thus deactivating the heater. Improper coupling of the heater to the circuit will also preferably cause the circuit to trip.
The circuit of the present invention can be implemented with a low parts count, using conventional components, thereby providing high reliability and low-cost.